Old chestnuts

Yes, things have moved on and it's usable. People are using it in the wild already. However, it's in an alpha state, things might be buggy, and things might change. While I can't officially recommend using it due to it's pre-release state, if you choose to do so, just realize that you're likely to need to help fix bugs in the framework as you're developing your own site.

Personally - being new to CI in general, stick with 3. It's a great framework that's stable and has been used to power many, many sites just fine.

I'm dying to see the upgrade path from CI 3 . Looking forward to seeing my facial expression when I see what has to be done haha.

Codeigniter is simply one of the tools you need to learn to be a successful developer. Always add more tools to your coding arsenal!